During gestation, a mother's microbiome shapes the immune system of her offspring, a new study in mice suggests. While it's known that a newborn's gut microbiota can affect its own immune system, the impact of a mother's microbiota on her offspring has largely been unexplored. Here, Mercedes Gomez de Agüero et al. infected the guts of pregnant mice with E.coli engineered to dwindle over time, allowing the mothers to become germ-free again around the time they gave birth. This temporary colonization of E.coli in the mother affected the immune system of her offspring; after birth, the offspring harbored more innate lymphoid and mononuclear cells in their intestines compared to mice born to microbe-free pregnant mothers. Similar results were seen when pregnant mothers were temporarily colonized with a cocktail of eight other microbes. An RNA analysis of offspring born to gestation-only colonized mothers compared with controls revealed greater expression of numerous genes, including those that influence cell division and differentiation, mucus and ion channels, and metabolism and immune function. By transferring serum from bacteria-colonized pregnant mice to non-colonized pregnant mice, the researchers found that maternal antibodies likely facilitate the transmission and retention of microbial molecules from a mother to her offspring. The results of this study add another surprising chapter to the growing body of literature surrounding the effects of the gut microbiota on immune functioning.